1. Field of the Invention
The present invention relates to spark plugs for use in various types of internal combustion engines.
2. Description of the Related Art
There are spark plugs for use in various types of devices and systems. For example, a spark plug is applied to an internal combustion engine mounted to a motor vehicle. Such a spark plug is a device for delivering electric current from an ignition system to a combustion chamber of then internal combustion engine in order to ignite air fuel mixture gas introduced into the combustion chamber. The spark plug has a center electrode and an earth electrode, in which a spark discharge gap is formed between the center electrode and the earth electrode. A projected part is formed on the earth electrode at the side which faces the center electrode, specifically, is close to the center electrode. For example, conventional techniques, disclosed in Japanese patent laid open publication No. JP 2009-054574 and disclosed in Japanese patent laid open publication No. JP 2009-054579, which show spark plugs having a projected part. The projected part is formed by deforming a part of base material of the earth electrode.
However, the structure of such a conventional spark plug having the projected part deformed at a part of the earth electrode causes a drawback of easily consuming the projected part of the earth electrode by a repetition of spark discharges. This increases the dimension of the gap of the spark discharge formed between the center electrode and the earth electrode in the conventional spark plug. As a result, this often shortens the life of the spark plug.
In order to solve the above drawback, one conventional technique has proposed an improved spark plug having an earth electrode with a projected part. The earth electrode and the projected part are plated with a thin metal layer in order to increase a wear resistance capability of the projected part and also to increase the life of the spark plug. Because the plating layer has in general a high hardness, it is possible to protect the surface of the projected part formed on the earth electrode of the spark plug.
Further, because the plating layer has a low thermal conductivity, it is difficult to discharge heat energy of the projected part of the earth electrode to outside, and as a result the temperature of the projected part of the earth electrode is thereby increased. This promotes activity of electrons around the projected part of the earth electrode, and increases the sparkability and ignitability around the projected part of the earth electrode in a spark plug.
However, the structure of the earth electrode in which the entire surface of the projected part is covered with the plating layer having a low thermal conductivity causes a drawback of it being difficult to quickly discharge thermal energy from the projected part having a high temperature of the earth electrode after spark discharge. This shortens the life of the projected part of the earth electrode, and easily consumes the life of the projected part of the earth electrode of the spark plug. As a result, this shortens the life of the spark plug.